Good sealing is required of food packages; that is, they must prevent moisture, microbes and other sources of contamination from coming into contact with the product, as well as prevent the product from penetrating the package. A commonly used package material is paperboard whose barrier properties have been improved by adding various coating layers onto the surface of the paperboard. In food packages, plastic layers are generally used, whose material is, for example, polyethylene (PE) or polyethelene terephtalate (PET). When preparing blanks for paperboard packages, sheets are cut from a coated paperboard web, and the cut edges thus form a weak point in view of the package, the so-called untrimmed edge that is not protected with a coating layer. This untrimmed edge must be sealed to provide sufficient impermeability for the packages and to prevent the absorption of the material to be packed into the paperboard as well as to prevent sources of contamination from coming into contact with the material to be packed. Commonly used methods for sealing the untrimmed edge include taping, heat-sealing and skiving, that is, the milling of the edge to be thinner, and the double-bending of this feather edge to seal the untrimmed edge.
Document EP 0 152 616 discloses a method and an apparatus for protecting a raw edge in paper laminates. In this method, the edge of the paper laminate is treated with a laser beam, wherein part of the open paperboard layer between the laminate layers is removed with the laser beam. After this, the laminate layers remaining in the edge are pressed together, and the edge is sealed by heating. The method can be used either continuously or for single sheets.
U.S. Pat. Nos. 5,801,243 and 4,931,031 disclose methods for protecting the raw edge of coated paperboard, based on skiving, that is, the thinning of the edge, followed by bending of the feather edge. The edge of the paperboard is thinned by cutting off a thin slice, leaving the coating layer and a thin layer of the paperboard, and the feather edge is then folded so that the raw edge of the paperboard is protected. The method disclosed in U.S. Pat. No. 4,931,031 is suitable for continuous sealing of a raw edge.
International publication WO 99/25548 discloses yet another method for the manufacture of package blanks with protected raw edges. In the method, the coating of the paperboard and the protecting of the edges is started in a continuous web, after which the web is cut into sheets. All the edges of the paperboard sheets are protected so that the coating layer on the surface of the sheets extends over the edge to be protected, and this outreaching strip is folded over the edge and sealed onto the edge. Finally, package blanks are formed of the paperboard sheets by sealing the edges of the sheets together.
Publication GB 1 013 656 discloses the seaming of two adjacent edges abuttingly together by using a coating layer on the surface of the paperboard and heat-sealing it onto the surface of the adjacent paperboard.
In the above-mentioned methods, the raw edge of the paperboard or paper is protected by utilizing coating layers already provided on the surface of the paperboard or paper. The edge of the paperboard or paper is trimmed so that the raw edge can be closed by folding and sealing the coating layer over the raw edge to be protected. Such methods require several different work stages before the raw edge is protected.